1. Field of the Invention
The present invention relates to a pitch conversion method and device, and in particular to a pitch conversion method and device for converting a pitch of an input signal into a desired (target) pitch in order to change e.g. a voice level or accent.
2. Description of the Related Art
Prior art examples [1] and [2] of the above-mentioned pitch conversion technology will now be described referring to FIG. 24.
Prior Art Example [1] (PSOLA method): FIG. 24
In this pitch conversion technology, a pitch conversion is performed by overlapping and adding waveforms of an input signal per pitch cycle in conformity with a target pitch (namely, the input signal is eventually expanded or contracted in the direction of time axis), and is generally called a PSOLA (Pitch-Synchronous Overlap and Add) method (see e.g. patent document 1).
FIG. 24 shows an example of the pitch conversion for contracting an input signal “In” in the direction of time axis by using the PSOLA method.
Namely, two waveforms W1 and W2 are firstly cut from the input signal “In” per pitch cycle T, and then window functions F1 and F2 are respectively applied to the cut waveforms W1 and W2 to adjust the amplitudes. In order to avoid discontinuity of waveforms at the boundary between an overlapped portion of the waveforms W1 and W2 by overlapping and adding which will be described later and the non-overlapped portion, the window functions F1 and F2 are set so that the sum of mutual contribution degrees may become “1” at the overlapped portion of the waveforms W1 and W2 as shown in FIG. 24.
Then, two waveforms (not shown) whose amplitudes are adjusted by the window functions F1 and F2 are overlapped and added to obtain the output signal “Out”.
In such a prior art example [1], waveforms after the pitch conversion may be deformed since waveforms whose phases are different from each other are overlapped. This deformation is notable especially when a pitch conversion ratio (namely, an expansion and contraction ratio of the input signal in the direction of time axis) is large, which leads to a degradation of sound quality.
In order to deal with this problem, a prior art example [2] has been already proposed as described herebelow:
Prior Art Example [2]: Not shown
In this pitch conversion technology, a linear predictive analysis is firstly performed to the input signal, so that the signal is separated into an envelope signal (formant component) and a residual signal (harmonics component). Then, a pitch conversion is performed only to the residual signal in the same way as the above-mentioned prior art example [1], so that the residual signal after the pitch conversion has been performed and the original envelope signal are synthesized by using a linear predictive coefficient calculated from the input signal.
Thus, the pitch conversion can be performed without affecting the envelope signal, and the above-mentioned waveform deformation due to the pitch conversion can be reduced, so that a degradation of sound quality can be avoided (see e.g. patent document 2).    [Patent document 1] Japanese Patent Application Laid-open No. 10-78791    [Patent document 2] Japanese Patent Application Laid-open No. 7-219597
While in the above-mentioned prior art example [2] the pitch conversion can be performed without deteriorating the sound quality of the input signal compared with the above-mentioned prior art example [1], there is a problem that the linear predictive analysis and the signal separation/synthesis require processing of large data throughput (calculation amount or the like).